ClearNonLiveTransitions has to hold on to non-map values.

src/objects.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 7381 matching lines @@
   for (int i = 0; i < length; i++) {
     fprintf(file, "%c", Get(i));
   }
 }
 
 
 // Clear a possible back pointer in case the transition leads to a dead map.
 // Return true in case a back pointer has been cleared and false otherwise.
 // Set *keep_entry to true when a live map transition has been found.
 static bool ClearBackPointer(Heap* heap, Object* target, bool* keep_entry) {
-  if (!target->IsMap()) return false;
+  if (!target->IsMap()) {
+    *keep_entry = true;

[Michael Starzinger, 2012/06/05 09:12:48]

I don't think this fix is completely correct, because in the ELEMENTS_TRANSITION
case we might have a slot in the backing store that is undefined. In this case
the backing store might be kept even though it is empty.

[Toon Verwaest, 2012/06/05 10:29:16]

Done.

+    return false;
+  }
   Map* map = Map::cast(target);
   if (Marking::MarkBitFrom(map).Get()) {
     *keep_entry = true;
     return false;
   } else {
     map->SetBackPointer(heap->undefined_value(), SKIP_WRITE_BARRIER);
     return true;
   }
 }
 
@@ skipping 5805 matching lines @@
   set_year(Smi::FromInt(year), SKIP_WRITE_BARRIER);
   set_month(Smi::FromInt(month), SKIP_WRITE_BARRIER);
   set_day(Smi::FromInt(day), SKIP_WRITE_BARRIER);
   set_weekday(Smi::FromInt(weekday), SKIP_WRITE_BARRIER);
   set_hour(Smi::FromInt(hour), SKIP_WRITE_BARRIER);
   set_min(Smi::FromInt(min), SKIP_WRITE_BARRIER);
   set_sec(Smi::FromInt(sec), SKIP_WRITE_BARRIER);
 }
 
 } }  // namespace v8::internal